To achieve high-capacity fiber optic communications, wavelength division multiplexing (WDM) technology is being adopted. In WDM, a tunable light source (TLS) capable of emission at different wavelengths is used. A wavelength monitor circuit is provided in a tunable light source to control the wavelength. In general, a wavelength monitor circuit is implemented by combination of a spectral filter (or a wavelength filter) having a periodic transmission spectrum and a photodiode (PD). A delay interferometer is used as such a wavelength filter. See, for example, U.S. patent application publication No. 2015/0085292 A1 and International Publication No. WO 2016/010528 A1.
FIG. 1 illustrates a configuration of conventional wavelength tunable light source. By having a light beam output from a semiconductor optical amplifier (indicated as “SOA1”) bounce between a resonator filter and the SOA1, light with a specific wavelength is amplified selectively. A portion of the amplified light is taken out from tap-1 and branched into two at tap-2. One of the branched light components is input to a wavelength filter, and the other is amplified by and output from another semiconductor optical amplifier (indicated as “SOA2”). A portion of the output light is received by a power monitoring photodiode “PDm”. The wavelength filter has a 3 dB coupler, a 90-degree hybrid coupler, and two waveguides extending at different lengths between the 3 dB coupler and the 90-degree hybrid coupler. Two light components output from the 90-degree hybrid coupler, with optical phases offset by 90 degrees from each other, are received at PD1 and PD2, respectively.
Photocurrents detected by the PD1 and the PD2 of the wavelength monitor circuit are denoted as I1 and I2, respectively, and photocurrent detected by the power monitoring photodiode “PDm” is denoted as Imon. FIG. 2 illustrates spectra of two current ratios (i.e., transmission spectra of the wavelength filter) as a function of wavelength. The current ratio of I1 to Pmon (I1/Imon) is indicated by a thin solid line and the current ratio of I2 to Imon (I2/Imon) is indicated by a bold line. In order to precisely control the wavelength so as to be consistent with the respective wavelength gridlines, it is preferable for the filtering spectrum to have a greater slope. The greater the slope, the more sensitive the wavelength control with more derivative gain. At each wavelength gridline, whichever of the current ratios I1/Imon and I2/Imon with a greater slope is selected, and the wavelength is controlled by adjusting heater current for the resonator filter so as to bring the current ratio to be the target value.
With the configuration of FIG. 1, the intensity of light received at the power monitor “PDm” varies depending on the state of the semiconductor optical amplifier “SOA2”, and the wavelength monitor circuit cannot achieve satisfactory accuracy. Another known structure of wavelength monitor circuit is illustrated in FIG. 3. A portion of light guided to the wavelength monitor circuit is branched at tap-3, and received by an additional photodiode PDmW for wavelength monitoring. The light received at PDmW is used as reference light.
In the configuration of FIG. 3, the additional tap-3 is required and the quantity of light received at PD1 and PD2 is reduced. If the split ratio (or coupling ratio) of tap-2 is adjusted for the purpose of securing an adequate quantity of light for wavelength monitoring, the intensity of light output from the SOA2 will decrease. Besides, the coupling ratio of tap-3 depends on the wavelength. In order to acquire an adequate quantity of light needed for the wavelength monitoring photodiode PDmW, the coupling ratio of tap-3 is determined on the basis of a wavelength that minimizes the branching ratio of light supplied from tap-3 to PDmW. This is because, at a wavelength that maximizes the branching ratio of light supplied from tap-3 to PDmW, the quantity of light received at PD1 and PD2 decreases. To compensate for the reduction in the quantity of received light, the coupling ratio at tap-2 toward the wavelength filter has to be increased. This results in increased optical loss, and the power level of light output from the light source decreases due to the optical loss. Moreover, another problem arises such that the controlled wavelength itself may shift and the wavelength filter becomes unstable, due to the wavelength dependency of the coupling ratio of tap-3.
There is a demand for a wavelength tunable light source using a wavelength monitor with stable operation and less optical loss.